- Carbon stored in organic and mineral soil horizons in forests gradually accumulates over time, resulting in higher stores of soil carbon in old growth than younger forests.
During stand development, part of the carbon sequestered by trees and other vegetation is transferred to the carbon stock of the soil in the form of litterfall and root turnover. Empirical studies, including chronosequences, suggest that organic matter or carbon accumulates in the forest ﬂoor as the stand ages but development of carbon stock deeper in the soil is slow (Peltoniemi et al., 2004Peltoniemi, M., Mäkipää, R., Liski, J., Tamminen, P. 2004. Changes in soil carbon with stand age – an evaluation of a modelling method with empirical data. Glob. Chang. Biol. 10(12):2078-2091. More, and references within). Old forests continue to accumulate carbon in forest floors and mineral horizons in the absence of fire (Entry and Emmingham, 1998Entry, J.A., Emmingham, W.H. 1998. Influence of forest age on forms of carbon in Douglas-fir soils in the Oregon Coast Range. Can. J. For. Res. 28:390-395. More, Giesen et al., 2008Giesen, T.W., Perakis, S.S., Cromack, K. 2008. Four centuries of soil carbon and nitrogen change after stand-replacing fire in the western Cascade Range of Oregon. Can. J. For. Res. 38: 2455-2464 More). However, Giesen et al. (2008)Giesen, T.W., Perakis, S.S., Cromack, K. 2008. Four centuries of soil carbon and nitrogen change after stand-replacing fire in the western Cascade Range of Oregon. Can. J. For. Res. 38: 2455-2464 More found that forest floor and surface mineral carbon and nitrogen pools stabilized or decreased some time between a stand age of 450 and 800 years, although the researchers could not explain this finding and recommended further study.
In Douglas-fir dominated stands in western Oregon, total carbon in the L, F, and H forest floor horizons and in organic material in the top 10 cm of mineral soil was higher in old growth forests (120-250 years old) than in second growth (50-80 years old) and young (15-30 years old) forests (Entry and Emmingham,1997). The second growth and young forests were burned prior to establishment. Differences were particularly great for total carbon in the mineral soil, where values were 115, 47, and 25 g C kg soil-1 for old, second growth, and young forests, respectively.